USB3500

DS00002103A-page 24

5.7.2

VBUS CONTROL

The USB3500 includes all of the Vbus comparators required for OTG. The VbusVld, SessVld, and SessEnd compara-

tors are fully integrated into the USB3500. These comparators are used to ensure the Vbus voltage is the correct value

for proper USB operation.

The VbusVld comparator is used by the Link, when configured as an A device, to ensure that the Vbus voltage on the

cable is valid. The SessVld comparator is used by the Link when configured as either an A or B device to indicate a

session is requested or valid. Finally the SessEnd comparator is used by the B-device to indicate a USB session has

ended.

Also included in the VBUS Control block are the resistors used for VBUS pulsing in SRP. The resistors used for VBUS

pulsing include a pull-down to ground and a pull-up to VDD3.3.

5.7.2.1

SessEnd Comparator

The SessEnd comparator is designed to trip when Vbus is less than 0.5 volts. When Vbus goes below 0.5 volts, the

session is considered to be ended and SessEnd will transition from 0 to 1. The SessEnd comparator is disabled when

the Suspendn = 0. When disabled, the SessEnd output is 0. The SessEnd comparator trip points are detailed in Table 4-

7.

5.7.2.2

SessVld Comparator

The SessVld comparator is used when the PHY is configured as either an A or B device. When configured as an A

device, the SessVld is used to detect Session Request protocol (SRP). When configured as a B device, SessVld is used

to detect the presence of Vbus. The SessVld comparator is not disabled with Suspendn and its output will always reflect

the state of VBUS. The SessVld comparator trip point is detailed in Table 4-7.

5.7.2.3

VbusVld Comparator

The final Vbus comparator is the VbusVld comparator. This comparator is only used when configured as an A-device.

In the OTG protocol the A-device is responsible to ensure that the VBUS voltage is within a certain range. The VbusVld

comparator is disabled when Suspendn = 0. When disabled the VbusVld will read 0. The VbusVld comparator trip points

are detailed in Table 4-7.

When the A-device is able to provide 8-100mA, it must ensure Vbus doesn’t go below 4.4 volts. If the A-device can pro-

vide 100-500mA on VBUS, it must ensure that Vbus does not go below 4.75 volts.

The internal Vbus comparator is designed to ensure that Vbus remains above 4.4 volts. If the design is required to sup-

ply over 100mA an external Vbus comparator or overcurrent fault detection should be used.

5.7.2.4

Vbus Pull-up and Pull-down Resistors

In addition to the internal Vbus comparators, the USB3500 also includes the integrated VBUS pull-up and pull-down

resistors used for VBUS Pulsing. To discharge the VBUS voltage, so that a Session Request can begin, the USB3500

provides a pull-down resistor from VBUS to Ground. This resistor is controlled by the DISCHRGVBUS pin. The pull-up

resistor is connected between VBUS and VDD3.3. This resistor is used to pull Vbus above 2.1 volts to indicate to the

A-Device that a USB session has been requested. The state of the pull-up resistor is controlled by the CHRGVBUS pin.

The Pull-Up and Pull-Down resistor values are detailed in Table 4-7.

5.7.2.5

Vbus Input Impedance

The OTG Supplement requires an A-Device that supports Session request protocol to have an input impedance less

than 100kohm and greater the 40kohm to ground. In addition, if configured as a B-Device, the PHY cannot draw more

then 150uA from Vbus. The USB3500 provides a 75kΩ nominal resistance to ground which meets the above require-

ments.

Note:

The OTG Supplement specifies a voltage range for A-Device Session Valid and B-Device Session Valid

comparator. The USB3500 PHY combines the two comparators into one and uses the narrower threshold

range.